In Tk::Pod::FindPods I just use the following subroutine to determine whether a module is a non-core module:

sub is_site_module {
    my $path = shift;
    if ($^O eq 'MSWin32') {
        return $path =~ m|[/\\]site[/\\]lib[/\\]|;
    }
    $path =~ /^(
                \Q$Config{'installsitelib'}\E
               |
                \Q$Config{'installsitearch'}\E
               )/x;
}
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